This invention relates in general to the field of light sources for applications in which color temperature and chromaticity characteristics are important factors. Such applications arise, for example, in the medical and dental fields. In particular, this invention relates to metal halide discharge lamps and techniques used in connection therewith to produce light sources having desired color temperature and chromaticity characteristics.
In a typical high-intensity discharge metal halide lamp, a pair of refractory metal electrodes are sealed into a quartz arc chamber or emission envelope spaced apart in relation to each other. Each electrode terminates at a tip and, together, the tips define an arc discharge path therebetween. The arc chamber contains a chemical fill of mercury, a metal halide, and a rare gas, such as argon, at a pressure of several Torr.
Light is emitted from the lamp when a discharge is produced between the electrode tips. At discharge, the temperature of the arc chamber rapidly increases, causing the condensed mercury and metal halide to vaporize. The mercury atoms and the metal atoms of the metal halide are ionized and excited, causing emissions of radiation at spectrums that are characteristic of the metals. This radiation is substantially combined within the arc chamber to produce a resultant light output. However, metal halides tend to emit radiation, at their dominant spectrums, in regions spatially removed from the discharge arc. This may result in a secondary light output at the dominant spectrum. For instance, the metal halide, sodium iodide, will emit an orange halo from the arc chamber.
The halo or ring produced by metal halides has been considered undesirable in many light source applications. Therefore, aperture stops have been used to physically block the halo emission before coupling the light output for its intended application. Rather than eliminate this halo, the present invention utilizes it to produce a light output having color temperature or chromaticity characteristics optimum for a given application.
In many light source applications, including those in the field of medical and dental instruments, a lamp may be operated in conjunction with an optical system that includes a concave reflector. The optical system is adapted to direct light, emitted from the lamp, to a desired target area for illumination. The primary goal for these optical systems has been to maximize the light output of the lamp. Color temperature or chromaticity characteristics of the system have not been a prime consideration, and, in most cases, have been ignored entirely.
One example of such a system is described in U.S. Pat. No. 3,700,881, entitled "Lamp and Reflector Assembly." The preferred embodiment described therein is an ellipsoidal reflector containing a metal halide high-intensity arc lamp. In manufacture, the arc lamp is adjusted so that the brightest part of the arc is exactly at the near focus of the reflector for maximum light output. Unlike the present invention, the system's color temperature or chromaticity characteristics are not optimized for the system's intended applications.
These characteristics are now an important consideration in the design of medical and dental instruments and operating and examining room lighting. For example, plastic surgeons prefer light that is as close to natural daylight as possible to observe the true colors of the patient's skin and any tumors or lesions growing thereon. In addition, designs for operating rooms have been proposed with light sources approximating natural daylight. The color temperature of these light sources have been specified between 5,500.degree. K and 6,000.degree. K.
Specific examples of these light sources are described in U.S. Pat. Nos. 4,072,856 and 3,588,488. U.S. Pat. No. 4,072,856 discloses a fixture having a multi-layered reflector. The reflector provides a spectral distribution of reflected light approximating daylight and having a color temperature of 5,500.degree. K. A shield is used to prevent direct illumination from the lamp Only light corrected by the reflector is emitted from the fixture. The fixture is a complicated article of manufacture, requiring careful deposition of compensating layers onto a substrate. The fixture also requires a light shield as an additional component. U.S. Pat. No. 3,588,488 discloses a similarly complicated fixture including various filters and multi-layered coatings for correcting the light emitted from a halogen lamp. The present invention, on the other hand, provides for a less complex and more efficient light source, as a result of direct illumination and the achievement of color correction without filtering.
In another effort, a metal halide lamp has been proposed in U.S. Pat. No. 4,528,478 purporting to achieve substantially "white" light by selecting the proper amount and combination of metal halides. However, this approach requires exceptional precision during the manufacture of the lamp to achieve repeatable performance. In addition, the lamp's color performance is extremely sensitive to the proportions of metal halides used. Therefore, normal consumption and condensation behavior of the metal halides, during lamp operation, will cause the color characteristics of these lamps to vary over time and from one lamp to another. On the other hand, the present invention provides a light source that is relatively insensitive to normal metal halide consumption and variations in condensation.